1. Field of the Invention
The present invention relates to a battery provided with a battery case of aluminum alloy, for example, in which a battery case body and a battery body are fitted to each other and are connected by laser welding.
2. Description of the Related Art
A large scale non-aqueous electrolytic secondary battery, which is used for an application of large capacity, has used a battery case of stainless steel. However, in recent years, in order to reduce the weight, the battery case of aluminum alloy has been used in many cases. The non-aqueous electrolytic secondary battery provided with a battery case of aluminum alloy is constructed as follows. Within a battery case body 1 in a shape of a long cylindrical vessel of aluminum alloy as shown in FIG. 10, a power generating element (not shown) and positive and negative terminals 3, 4 connected to the electrodes thereof are housed. In this state, an oval battery cover 2 of aluminum alloy is fit in an upper end opening of the battery case body 1 and its periphery is sealed by welding as shown in FIG. 11. In this case, the peripheral edge of the oval battery cover 2 is bent upwards to constitute a bonding area 2a which is fit along a bonding area 1a of the upper end opening of the battery case body 1. As shown in FIG. 13, the respective bonding areas 1 a and 2a of the battery case body 1 and battery cover 2 are connected by welding, thereby completing a battery case. In the case of the battery case of stainless steel, TIG welding is used, whereas in the case of the battery case of aluminum alloy, laser welding is used.
Incidentally, as shown in FIG. 10, the battery cover 2 has terminal holes 2b and 2c through which positive and negative terminals 3 and 4 protrude and an injection hole 2d for injecting an electrolytic solution into the battery case. It should be noted that these holes are made previously. Further, as shown in FIG. 11, after the battery cover 2 is welded, the terminal holes 2b and 2c are closed by screwing nuts to the positive and negative terminals 3 and 4 from the battery cover 2. The injection hole 2d is closed by a setscrew, for example, after the electrolytic solution is injected.
Meanwhile, the laser welding is performed in such a manner that a body is irradiated with laser light so that the body absorbs the energy of the laser light and hence is heated and molten. In this case, before being molten, the aluminum alloy used for the battery case body 1 and battery cover 2 decreases the absorption rate since it reflects almost the entirety of the laser light, whereas after being molten it increases the absorption rate. Therefore, lower laser power cannot easily melt the aluminum alloy. However, increased laser power abruptly heats the aluminum alloy after being molten. As a result, the aluminum alloy thus molten is scattered so that pinholes or cracks may be generated in the bonding areas 1a and 2a. 
Thus, it is difficult to adjust the laser power when the battery case of aluminum alloy is laser-welded. If the laser power is increased, there was a problem that the pin-holes and cracks may be generated in the bonding areas 1a and 2a between the battery case body 1 and battery cover 2.
It is an object to provide a battery in which a battery case can be easily laser-welded with low laser power. According to a first aspect, the present invention provides a battery provided with a battery case in which with a battery cover of aluminum or aluminum alloy fit into a battery case body of aluminum or aluminum alloy, their bonding areas are connected by laser welding. In the battery, the bonding areas of the battery case body and battery cover are mated with each other and the thickness of their bonding areas is made smaller than that of the remaining areas of the battery case body and battery cover.
Aluminum or aluminum alloy has a very high thermal conductivity. Therefore, if the bonding areas of the battery case body and battery cover has a large thickness, when energy is absorbed from irradiated laser light, heat diffuses soon into the environment. As a result, the heat cannot be concentrated into the irradiated portion. However, in accordance with the first aspect of the present invention, the thickness of each of the bonding areas is small so that the heat generated from the irradiation of laser light is difficult to conduct into the environment. Thus, the heat can be concentrated into the irradiated portion. Accordingly, even if the aluminum or aluminum alloy of the bonding areas does not absorb the energy of laser light before it is molten, the heat generated by the laser light can be effectively used, thereby surely melting the aluminum or aluminum alloy. Further, this permits laser welding to be performed with reduced power. For this reason, the fear that pin-holes and cracks are generated in the bonding areas by excessive heating after being molten can be eliminated.
Preferably, the thickness of each of the bonding areas is 0.5 mm or larger in order to assure the strength of a battery case, and 1.2 mm or smaller in order to improve the melting in laser welding. Particularly, such limitation is effective for a battery having capacitance of 10 Ah or larger.
According to the second aspect, the present invention provides a battery provided with a battery case in which with a battery cover of aluminum or aluminum alloy fit into a battery case body of aluminum or aluminum alloy, their bonding areas are connected by laser welding. In the battery, the oxygen content of aluminum in the bonding areas of the battery case body and battery cover is 1000 ppm or lower.
When aluminum or aluminum alloy is left in the air, its surface is oxidized to form an oxide covering film. Such an oxide covering film has a high melting point. Therefore, if the temperature resulted from the irradiation of the laser light is too low, this oxide covering film hinders aluminum or aluminum alloy in the bonding areas from being molten. However, in accordance with the second aspect of the present invention, the oxygen content of the aluminum alloy of the bonding areas can be reduced to 1000 ppm or less so that the oxide having a high melting point can be removed substantially completely. Thus, the aluminum alloy of the bonding areas can be easily molten. Accordingly, even if the aluminum or aluminum alloy of the bonding areas does not absorb the energy of laser light before it is molten, the heat generated by the laser light can be effectively used, thereby surely melting the aluminum or aluminum alloy. Further, this permits laser welding to be performed with reduced power. For this reason, the fear that pin-holes and cracks are generated in the bonding areas by excessive heating after being molten can be eliminated.
According to the third aspect, the present invention provides a battery provided with a battery case in which with a battery cover of aluminum or aluminum alloy fit into a battery case body of aluminum or aluminum alloy, their bonding areas are connected by laser welding. In the battery, a cladding material of aluminum or aluminum alloy having a lower melting point than that of the case body is bonded to at least a bonding face of at least one of the battery case body and battery cover.
The battery case of aluminum or aluminum alloy is generally made of a material having a high melting point in order to assure its sufficient strength. However, aluminum or aluminum alloy having a lower melting point than that of the case body is located between the respective bonding areas of the battery case body and battery cover in order to assure their strength. Thus, such aluminum or aluminum alloy having a low melting point can be easily molten by irradiation of light. Accordingly, even if the aluminum or aluminum alloy of the bonding areas does not absorb the energy of laser light before it is molten, the aluminum or aluminum alloy having a lower melting point is molten precedently to enhance the abortion rate of laser light. This permits the aluminum or aluminum alloy having a higher melting point to be molten. Further, this permits laser welding to be performed with reduced power. For this reason, the fear that pin-holes and cracks are generated in the bonding areas by excessive heating after molten can be eliminated.
According to the fourth aspect, the present invention provides a method of manufacturing a battery provided with a battery case in which with a battery cover of aluminum or aluminum alloy fit into a battery case body of aluminum or aluminum alloy, their bonding areas are connected by laser welding. The method comprises the steps of: forming a notch at an outer edge of a bonding face of at least one of the battery case body and battery cover and constituting a groove due to the notch when the battery cover is fit into the battery case body; and irradiating the groove with laser light for laser welding.
In accordance with the fourth aspect, the groove between the bonding areas of the battery case body and battery case cover is irradiated with laser light. The absorption rate of the laser light can be increased, thereby surely melting the aluminum or aluminum alloy. Further, this permits laser welding to be performed with reduced power. For this reason, the fear that pin-holes and cracks are generated in the bonding areas by excessive heating after molten can be eliminated.
According to the fifth aspect, the present invention provides a method of manufacturing a battery provided with a battery case in which with a battery cover of aluminum or aluminum alloy fit into a battery case body of aluminum or aluminum alloy, their bonding areas are connected by laser welding. The method comprises the steps of: subjecting laser-light-irradiated surface at the bonding areas of the battery case body and the battery cover to surface processing so that the surface roughness ranges from 1 xcexcm to 100 xcexcm; and irradiating the bonding areas with laser light for laser welding when the battery cover is fit into the battery case body.
In accordance with the fifth aspect of the present invention, the laser-light-irradiated surface at the bonding areas of the battery case body and the battery cover is subjected to surface processing. Thus, the absorption rate of the laser light can be enhanced, thereby surely melting the aluminum or aluminum alloy. Further, this permits laser welding to be performed with reduced power. For this reason, the fear that pin-holes and cracks are generated in the bonding areas by excessive heating after molten can be eliminated.